Well, to be honest, a hereditary monarchy does make sense where the planet is actually owned by the original settlers - if one family owns enough of the land, then a monarchy would be entirely appropriate, as would lords and other nobility, depending on who owns the land they have granted to them - if it's still (legally) in the name of the monarch, but they've granted it to you, they're still only (legally) loaning it to you - a monarch can take it back at any point, but would need an excuse or face pressure from other nobles. Commoners would likely be paid to come and settle the planet (while on a more democratic planet, settlers would probably need to pay for their own transport and supplies) and granted supplies with which to build their homes, in exchange for not expecting any land grants.
Keeping a monarchy would be possible, mainly by limiting military power in the hands of the commoners and by having an effective military, but one where the power is split up to prevent a coup and, after a while, people would get used to it. In some ways, as long as the nobility and the monarchy are actually doing their jobs and looking after the people below them, then the commoners may actually prefer that setup - they don't need to be bothered with anything over local politics and can get on with their lives. Democracy as it is today isn't that much different - you still get some faceless government ruling over you, it's just that you get the impression that you help to make the rules, whereas the influence you get to exert isn't as much as we'd sometimes like (or even like to admit). There are times, even, when not having bickering at the top is actually beneficial.
Of course, where all this comes tumbling down is where you have a monarch who is either heavy-handed, neglectful of his duties to his people or just plain greedy, so causes trouble for all involved.
And, of course, any planetary government, after a while, will come into contact with another and, sooner or later, a conflict will ensue. The winner will usually set up their own form of government on the planet they conquer and so monarchies, republics and so on, become empires (even if by another name). Larger empires will swallow up lone planets and smaller empires. Repeat a few hundred times and you end up with larger empires which can't be stopped, even by makeshift alliances...
Then, of course, you have the constitutional monarchies - having a titular head to the government can be a good thing too - someone to provide a bit of continuation without having too much power... of course, they're also a drain on the national wallet, so need to be balanced.
Lastly, a republic was what Rome had... and then one Senator got too much power, gathered the army to himself and made himself Emperor (Imperator) over the weak Senate... which was eventually abolished. No form of government is superior, merely different - even communism was originally set up as being beneficial to all, but much like any other, the original concept and intent was corrupted. The US was founded on equality for all, but you have a nobility in all but name in the form of the corporate families - inherited fortunes that create power through wealth and wealth through power... are you so sure that, if given the opportunity, a multi-billionaire wouldn't actually buy the rights to a whole new planet and call himself King if he thought he could get away with it? I'm not.